The invention relates to a vacuum brake booster having a vacuum chamber and a working chamber separated from one another by a movable wall, a control valve, which comprises a housing coupled workingly to the movable wall and which to achieve a pressure difference at the movable wall is capable of controlling the supply of atmospheric pressure or above-atmospheric pressure to the working chamber in dependence upon the displacement of an actuating piston, and having an emergency braking aid comprising a permanent magnet and an armature, which cooperates with the permanent magnet and is resiliently biased counter to actuating direction and during an emergency braking operation is drawn into abutment with the permanent magnet, with the result that the control valve is held open for the supply of atmospheric pressure to the working chamber.
Vacuum brake boosters have been known for some time and millions are in use for boosting the actuating forces of a vehicle hydraulic brake system and therefore keeping the actuating forces at a level which is comfortable for the driver of a vehicle. Likewise known are so-called emergency braking aids, which are frequently also referred to as xe2x80x9cbrake assistsxe2x80x9d. These are devices which, given substantially the same actuating force, in an emergency braking situation provide a driver with increased braking power.
Emergency braking aids may be divided into electro-magnetically actuated and mechanically actuated systems. For reasons of cost, the use of a mechanical system is frequently desirable.
A vacuum brake booster having such a mechanical emergency braking aid is known, for example, from WO 00/07862, and corresponding U.S. Pat. No. 6,505,539 B2 which is incorporated by reference herein. Said vacuum brake booster has a vacuum chamber and a working chamber separated from one another in a pressure-tight manner by a movable wall. A control valve, which has a housing coupled workingly to the movable wall, comprises an atmospheric valve seat, which to achieve a pressure difference at the movable wall is capable of controlling the supply of atmospheric pressure to the working chamber in dependence upon the displacement of an actuating piston, coupled to an input element of the brake booster.
For improved boosting of the braking force during emergency braking operations, a mechanical emergency braking aid is disposed in the housing of the control valve. The emergency braking aid comprises an armature, which cooperates with a permanent magnet and is rigidly connectable in actuating direction to the actuating piston by means of a stop formed on the actuating piston. The armature is resiliently biased counter to the actuating direction of the brake booster towards the stop formed on the actuating piston and, in the initial position of the control valve, is held at a first distance from the permanent magnet. In the course of an approach towards the permanent magnet the armature, when it is less than a previously defined second distance, which is smaller than the first distance, from the permanent magnet is drawn by the permanent magnet counter to the resilient bias force acting upon the armature and with simultaneous cancellation of its, in actuating direction, rigid coupling to the actuating piston into abutment with the permanent magnet. Consequently, the atmospheric valve, whose valve seat is formed integrally with a sleeve-shaped extension coupled rigidly to the armature, is held open to the maximum extent. The maximum possible pressure difference then builds up, with the result that the maximum possible boosting force of the brake booster is achieved.
After an emergency braking operation the armature has again to be detached from the permanent magnet. This is effected by means of a bar, which is rigidly connected to the actuating piston, extends at right angles to a longitudinal axis of the actuating piston and projects through a recess of the sleeve-shaped extension of the armature, which extension extends counter to actuating direction. During a return stroke of the input element of the brake booster, the actuating piston as well is displaced counter to actuating direction. The bar coupled to the actuating piston in said case comes into abutment with an end of the recess of the sleeve-shaped extension facing the vacuum chamber, so that the forces displacing the actuating piston counter to actuating direction are introduced into the sleeve-shaped extension coupled to the armature. As said forces exceed the force required to detach the armature from the permanent magnet, the displacement of the actuating piston counter to actuating direction, which is associated with a reduction of the actuating force summoned up by the driver, leads finally to a detaching of the armature from the permanent magnet.
Because of the displacement of the actuating piston counter to actuating direction which is necessary to detach the armature from the permanent magnet, the stop formed for the armature on the actuating piston has also moved away from the armature by the displacement distance of the actuating piston. After the detachment of the armature from the permanent magnet, the armature is consequently accelerated along the displacement path of the actuating piston by the bias force acting counter to the actuating direction of the brake booster upon the armature. The armature therefore impacts at high speed against the stop formed on the actuating piston. Said impact of the armature on the stop provided for the armature produces a clearly perceptible, obtrusive cutout noise.
Operating positions of the brake booster are additionally possible, in which, prior to the impact of the armature on the stop formed on the actuating piston, the valve seat formed on the free end of the sleeve-shaped extension of the armature impacts on the associated valve sealing element of the control valve and pulls the latter abruptly open. As a result, the connection between the working chamber and the vacuum chamber is opened and the braking force boost summoned up by the brake booster is abruptly reduced, which manifests itself to the driver in the form of an unpleasant force impact on the brake pedal.
The object of the invention is to provide a vacuum brake booster with mechanical emergency braking aid, which presents improved operating behaviour.
Proceeding from a vacuum brake booster of the type initially described, said object is achieved according to the invention by provision of a coupling device, which allows detachable coupling of the armature to the actuating piston.
By means of the coupling device according to the invention the movement of the armature counter to actuating direction relative to the actuating piston may be influenced in order to prevent an uncontrolled relative movement between armature and actuating piston as a result of detaching of the armature from the permanent magnet. The coupling device makes it possible, after the detachment of the armature from the permanent magnet, to prevent an acceleration of the armature counter to actuating direction or at least reduce such an acceleration in such a way that the cutout problems observed in prior art vacuum brake boosters no longer arise in vacuum brake boosters according to the invention.
Coupling is preferably effected as a consequence of a return stroke movement of the actuating piston, i.e. the coupling device may be activated by a return stroke movement. Coupling may be effected before or after the armature was pulled off the permanent magnet.
The coupling of armature and permanent magnet may be effected e.g. by means of frictional, non-positive or positive engagement. Preferably, the coupling device is designed in such a way that a coupling is effected only after a specific return stroke of the input element of the vacuum brake booster but still before detachment of the armature from the permanent magnet. Armature and actuating piston may be coupled both directly and indirectly to one another. An indirect coupling of actuating piston and armature is possible, for example, by coupling the actuating piston to a further component of an armature subassembly also comprising the armature. Thus, for example, the actuating piston may be coupled to a sleeve-shaped extension of the armature, which extension extends counter to actuating direction.
According to a first embodiment of the invention, the coupling device takes the form of a detent device, which allows a coupling of armature and actuating piston by means of a detent connection. The detent connection preferably comprises a first detent element and a complementary second detent element, which cooperate in order to couple armature and actuating piston. The detent connection is preferably formed and released by moving the first detent element in radial direction relative to a longitudinal axis of the vacuum brake booster. The first detent element is in said case advantageously biased in radial direction, i.e. either radially inwards or radially outwards.
According to a first variant of the first embodiment, the first detent element takes the form of a snap-action hook or snap ring, which cooperates with a second detent element e.g. in the form of an indentation or opening. The snap-action hook is advantageously disposed on the end of a substantially axially extending swivelling arm so that the snap-action hook is movable by means of a swivelling motion in radial direction. The snap-action hook may be integral with the swivelling arm. In this manner it is possible, for example, to manufacture the snap-action hook by bevelling the swivelling arm in certain areas.
The swivelling arm may be rigidly coupled to a further arm which allows deflection of the swivelling arm. Consequently, the further arm allows an actuation of the swivelling arm since a deflection of the further arm is transmitted to the swivelling arm. The deflection of the further arm is preferably the result of a cooperation of the further arm with a stop movable relative to the further arm. The stop may be formed in a housing of the vacuum pressure brake booster, for example.
The swivelling arm and the further arm may be manufactured separately and connected by welding, for example, in order to simplify manufacture of the coupling device and to avoid complex bending processes. It is advantageous in such a coupling device to attach the swivelling arm rather than the further arm in pivoting manner to the actuating piston. Such a construction allows an improved force introduction into the actuating piston since the retaining forces summoned up by the swivelling arm in the coupled position can directly be transmitted to the actuating piston.
The snap-action hook, i.e. the first detent element, may have an inclined surface cooperating with a border of the corresponding opening, i.e. the complementary detent element. In the extreme case, the snap-action hook may be embodied by said inclined surface. Measurement tolerances can be compensated by the inclined surface since it allows an always play-free cooperation of the snap-action hook with the opening provided for the snap-action hook.
According to a second variant of the first embodiment, the first detent element is spherical or annular in shape and cooperates with a second detent element in the form of a bevel in such a way that, depending on the axial position of the first detent element relative to the bevel, the armature is coupled to the actuating piston or the coupling is released.
In the two previously described variants of the first embodiment, the first detent element and the second detent element may be provided in each case both in the region of the armature and in the region of the actuating piston. Preferably, however, the first detent element is disposed in the region of the actuating piston and the second detent element is formed in a sleeve-shaped extension of the armature.
According to a second embodiment of the invention, the coupling device takes the form of a clamping device. In the second embodiment, the coupling of the armature to the actuating piston is consequently effected by a clamping joint.
The clamping device may comprise a clamping element, which is capable of generating a clamping force in radial direction, i.e. in relation to a longitudinal axis of the vacuum brake booster radially inwards or radially outwards. Preferably, the clamping element has, in relation to axial relative movements between armature and actuating piston in and counter to actuating direction, asymmetrical clamping properties. In other words, depending on the direction of the axial relative movement between armature and actuating piston, a clamping force of differing intensity is generated by the clamping element. The clamping element is advantageously designed in such a way that it generates a clamping force at least when the armature moves counter to actuating direction relative to the actuating piston.
According to a first variant of the second embodiment of the invention, the asymmetrical clamping properties are realized by means of a bevel, which cooperates with the clamping element. Thus, it is possible to design the clamping element in such a way that, as a result of a relative movement between armature and actuating piston in axial direction, it runs off along the bevel and therefore generates a radially effective clamping force. The clamping element advantageously takes the form of a rolling body, e.g. a ball or ring. The bevel may be formed both in the region of the armature, e.g. radially at the inside of a sleeve-shaped extension of the armature, and in the region of the actuating piston.
According to a second variant of the second embodiment of the invention, for realizing asymmetrical clamping properties a clamping arm is provided, which extends obliquely relative to the longitudinal axis of the brake booster and is preferably biased in radial direction. The clamping arm is preferably rigidly connected either to the actuating piston or to the armature or a component of the armature subassembly and comprises an end portion, which under initial tension contacts either the armature and/or the armature subassembly or the actuating piston. The asymmetrical clamping properties of the clamping arm are attributable to its oblique position, i.e. the introduction of force at a specific angle e.g. from the armature into the actuating piston or vice versa.
Both in the first and in the second embodiment of the invention an axially movable opening sleeve may be provided, which allows a releasing of the coupling between armature and actuating piston. The opening sleeve is preferably disposed radially at the inside relative to a sleeve-shaped extension of the armature extending counter to actuating direction and is freely movable in axial direction relative to the sleeve-shaped extension. The releasing of a detent coupling between armature and actuating piston may be effected, for example, in that the opening sleeve cooperates with at least one of two complementary detent elements in such a way that the detent connection is cancelled. A clamping joint may be released by moving the opening sleeve in such a way in axial direction relative to a biased clamping arm that a biased end portion of the clamping arm is brought into abutment with the freely movable opening sleeve.
According to a third embodiment of the invention, the coupling device takes the form of a supporting device, which for coupling the armature to the actuating piston allows a supporting of the armature counter to actuating direction against the actuating piston. The supporting device preferably comprises a swivelling supporting element, which for coupling armature and actuating piston is swivelled into a supporting position.
Independently of the concrete construction of the coupling device, at least one stop may be formed on the actuating piston, which stop is disposed in actuating direction upstream of the armature and cooperates counter to actuating direction with the armature. The detaching of the armature from the permanent magnet is then effected no longer, as in prior art, by means of a bar but by means of the stop formed on the actuating piston. The bar is therefore relieved of its function with regard to detaching of the armature from the permanent magnet, thereby reducing the constructional requirements of the bar. The bar may therefore perform additional functions or, in extreme cases, even be dispensed with entirely, without impairing the process of detaching the armature from the permanent magnet.
The stop formed on the actuating piston preferably cooperates directly with the armature counter to actuating direction. Thus, the stop may cooperate e.g. with an end face of the armature facing the vacuum chamber of the brake booster. The end face of the armature cooperating with the stop is preferably not identical with the end face of the armature which comes into abutment with the permanent magnet. The end face of the armature cooperating with the stop may be disposed, for example, offset axially in or counter to actuating direction relative to the end face of the armature cooperating with the permanent magnet.
The vacuum brake booster advantageously comprises an actuating piston, on which on the one hand the previously described stop for the armature is formed and which on the other hand is additionally coupled to a bar, which extends at right angles to the longitudinal axis of the actuating piston. The bar may perform different functions. Thus, it is conceivable to limit the mobility of the actuating piston in axial direction by means of the bar. To said end, the bar may cooperate with a stop for the bar which is formed e.g. on the housing of the brake booster. The mobility of the actuating piston is advantageously limited at least counter to actuating direction by the interplay between the bar coupled to the actuating piston and the stop provided for the bar.